The Double Stars of Collinder 65, Part One: Σ 730, S 478, and Σ 697

Located north of the head of Orion and south of the horns of Taurus is a large open cluster known as Collinder 65 (CR 65), which is unlabeled on some star charts. You’ll find it about five degrees north of Lambda (λ) Orionis, stretched out across the Orion-Taurus border. (Lambda (λ) Orionis, a beautiful multiple star better known as Meissa, is at the center of another Collinder open cluster, CR 69).

Stellarium screen image with labels added, click to enlarge.

The diameter of CR 65 varies, ranging from 140’ (2.3°) to 220’ (3.7°), depending on which source you consult. At any rate, it’s noticeably larger than its Orion relative, CR 69.

Stellarium screen image with labels added, click for a larger view.

Both clusters are enticing binocular objects and rival one another in aesthetic appeal. Whereas CR 69 is dominated by the beautiful cream-colored white light of Meissa, CR 65 is illuminated by a handful of fifth and sixth magnitude Flamsteed-numbered stars. Scattered within that steady starlight are half a dozen multiple stars beckoning to the yearning eyes of a double star connoisseur.

The six double stars we’re going to look at are identified here in tantalizing turquoise. (Stellarium screen image with additional labels, click to enlarge).

The image above is a close approximation of what you would see in an 8×50 finder. In order to find your way around and have a sense of scale, it helps to know the distances between the various points, as well as the direction from star to star.   Those morsels of information were invaluable later when I navigated from one star to the next while looking through an eyepiece.  Below is the same image as above, but with the distances and directions added:

This is an erect image chart, which will match the orientation you see in an RACI (right angle,correct image) 8×50 finder. (Stellarium screen image, labels added, click to enlarge).

For use at the eyepiece of a refractor or SCT, here’s a mirror-reversed image of the same scene:

Stellarium screen image, labels added, click to enlarge:

We’ll start with Σ 730, which is located at the center of the eastern edge of CR 65.

Σ 730       HIP: 25950    SAO: 94630
(AB is also H III 93, H N 124, and Sh 58; Aa, Ab is OCC 999)
RA: 05h 32.2m   Dec: +17° 03’
Magnitudes  Aa, Ab: 7.30, 7.30   AB: 6.06, 6.44
Separations  Aa, Ab: 0.10”          AB: 9.60”
Position Angles  Aa, Ab: ?????   AB: 141° (WDS 2013)
Distance: 2991 Light Years (Simbad)
Spectral Classifications: “A” and “B are both B7

There was no doubt about the color of the AB pair – pure white. Not included among the components of Σ 730 is the eleventh magnitude star located about 1.5’ southeast of AB at about 155 degrees. (East & west are reversed here to match the refractor view, clicking on the sketch will enlarge it).

This is a pair of stars that received attention from all the well-known double stars observers of the late 18^th and early 19^th centuries: William Herschel, John Herschel, James South, and F.G.W. Struve.  It began with William Herschel seemingly confusing this pair of stars with 117 Tauri (source, scroll down to sixth title):

His position angle (which translates to 142° 27’) and separation for the two stars are reasonably close . However, his identification of this pair as 117 Tauri is an error which was discovered in 1821 by John Herschel and James South when they went in search of 117 Tauri and found it was a single star. Their discussion below is very clear and easy to follow (source, scroll down to last title):

Click to enlarge.

If you read the quotes from Herschel’s observing notes carefully, which are just past the mid-point of the page, it’s clear that William Herschel was using 117 Tauri as a reference point to reach the star now identified as Σ 730.  Herschel frequently included a Flamsteed-numbered star as a reference point in the first line of his observations, but in this case he left out the distance between 117 Tauri and the double star he observed in the published version, which led to John Herschel’s and James South’s search for it.  Fortunately, they had access to William Herschel’s notes.

Prior to the John Herschel-James South observation, William Herschel apparently observed this pair of stars a second time in 1800 and cataloged it again as H N 124 (source):

That observation is rather difficult to follow.   Apart from the coordinates, which match reasonably closely with those of Σ 730, his magnitudes of 9.9 don’t match at all, and I haven’t yet figured out what the Orion reference in the second line refers to.  I’m not all that convinced the pair of stars he described in 1800 is the same pair he saw in 1782. Nevertheless, H N 124 is treated as a duplicate of H III 93 by S.W. Burnham in his 1906 catalog, and the WDS refers to H N 124 in its notes on Σ 730.

Next, we’ll traverse the middle of CR 65 in search of S 478, which is on the west side of this cluster at a distance of 1° 52’. You can see by the 280° position angle I included on the last chart (here’s the erect image, and the mirror image) that we’re going to move almost due west with a very slight inclination toward the north.  Using 5.77 magnitude 117 Tauri as a stepping stone should get you to S 478 with little problem.

S 478      HIP: 25278    SAO: 94526
(AB is also H V 110; AC is WNO 52)
RA: 05h 24.4m   Dec: +17° 23’
Magnitudes   AB: 5.06, 8.79    AC: 5.06, 7.88
Separations  AB: 106.70”        AC: 705.20”
Position Angles   AB: 271°  (WDS 2011)   AC: 252° (WDS 2010)
Distance: “A” is 46.9 Light Years, “C” is 45.9 LY  (Simbad)
Spectral Classifications:  “A” is F8, “B” is K0, “C” is K4

The S 478 trio is a captivating visual delight. The white primary dominates the view, but the very slight hints of reddish-orange in the ninth magnitude “B” component and the wide eighth magnitude “C” component adds a touch of magic to the scene, and the 7.6 magnitude white light of SAO 94531 adds an extra touch of luster. (East & west reversed once more, click on the sketch to improve the view).

The AB pair was measured by Sir William Herschel during his observation of November 13^th, 1782 (source, scroll down to sixth title):

His Flamsteed number, 111 Tauri, is correct in this case, but the separation he recorded is off considerably, which was noticed by James South when he observed the AB pair on January 17^th and February 2^nd, 1825 (source):

On the last line of his observation, South refers to the minor change in position angle of the primary and secondary as being too small to account for the large difference in his and Herschel’s separation measures. Those position angles translate to 273° 48’ for Herschel and 271° 17’ for South, and they’re the first hint of significant motion taking place in one of the two stars.

As it turns out, the primary has a rather high rate of proper motion, which is partly attributable to its relatively close distance of 46.9 light years from us.  The “C” component, cataloged in 1897 as WNO 52, has a very similar rate of proper motion, and is located at a comparable distance, 45.9 light years.  The Aladin photo below shows the rates of proper motion of all three of the S 478 components:

Note that this in an erect image view, so east and west are opposite of what is shown in the sketch above of S 478. Click for a larger view.

Click to enlarge.

The similar motions of “A” and “C” are obvious, which in combination with their almost identical distances from us, has led to the conclusion the probability of their being physically related is close to 100%. The AB pair, on the other hand, is an optical pair, which is evident from the different rates of motion and direction of “A” and “B”.

The “C” component was added to S 478 in 1897 by the Washington Naval Observatory, which is the source of the WNO identifier assigned to AC. Also, there’s an eleventh magnitude star (TYC 1300-355-1) located 32” from “C” at a PA of 136°, for which there is no proper motion data, so it may or may not be moving in tandem with S 478 “A” and “C”.

You may have also noticed on the image above that S 478 “A” is labeled “BY Dra”, which is a reference to a class of variable star. The AAVSO (American Association of Variable Star Observers) designation for S 478 “A” (or 111 Tauri) is V1119 Tau.  Their data on the star shows a magnitude range of 4.98 to 5.02, so don’t hold your breath in anticipation of a wild swing in brightness.   The WDS data (shown above at right) also includes a note that S 478 “A” is a spectroscopic binary, which may account for its slight change in magnitude level.

So as you can see, hidden behind the visual appeal of the three stars of S 478, there are several layers of intriguing detail.

To get to our third stellar destination, Σ 697, we’ll move south and very slightly west a distance of 42.5’ to 6.1 magnitude 110 Tauri, and then continue due south another 39’ to our goal.  (Here’s the erect image again, and the mirror image).

Σ 697       HIP: 25207    SAO: 94512
RA: 05h 23.5m    Dec: +16° 02’

Identifier		Magnitudes	Separation	PA	WDS
STF 697	AB:	7.27,   8.10	25.90″	286°	2015
WAL 38	AC:	7.27, 10.83	97.90″	284°	2012
SMR 3	AD:	7.27, 10.07	249.30″	285°	2012
SMR 27	AE:*	7.27, 12.00	163.00″	290°	2012

Distance: 874 Light Years (Simbad)
Spectral Classifications: “A” is B8, “B” is A, “D” is B9

Note: The magnitude of E has been changed to 13.89; see comment below in red.

When I first looked at the data on Σ 697, I was immediately intrigued by the similar position angles of all five of the components. And even though I was prepared for what I saw, I was still impressed by the sheer unlikely beauty of the configuration.  Even 7.50 magnitude SAO 94498 and 8.53 magnitude HIP 25287 managed to line up with the components of Σ 697:

“A” and “B” caught my eye first, but it didn’t take more than a few seconds before “C” and “D” popped into view. I never did catch sight of twelfth magnitude “E”, which puzzled me since it should have been within reach of my five inch refractor. Both “A” and “B” are white, as are SAO 94498 and HIP 25287. (East & west reversed to match the refractor view, click on the sketch to get a better look at “C” and “D”).

I had a suspicion the 10.07 magnitude shown in the WDS for “E” was too bright, so I pulled up an Aladin photo in order to first make sure the star existed. What I found was a reddish-orange star, which partially explained why it was difficult to see. I never was able to come up with a spectral classification for that star, but I did find magnitudes for it in both the UCAC4 and the Nomad-1 catalogs. UCAC4 shows a visual magnitude of 13.885 for “E”, and Nomad-1 lists it at a visual of 13.710, which explains why I couldn’t detect any sign of it in my five inch refractor. That component is a candidate for a magnitude change, so I’ll be in contact with Bill Hartkopf at the WDS.  (NOTE: The magnitude of “E” has been changed to 13.89 as of 2/13/2015 and the UCAC4 proper motion data has also been added).

Click on the image and the data will be easier to read!

Click to enlarge.

The AB pair, which seems to have eluded William Herschel, was discovered in 1828 by F.G.W. Struve, as shown at the left (source).  As the excerpt shows, there’s been little change in the position angle and separation since 1828, which matches well with Simbad’s proper motion data on the two stars: -001 -008 (.001”/yr west, .008”/yr south) for the primary, and -003 -004 (.003”/yr west, .004”/yr south) for the secondary.  In fact, there’s little movement among all four of the stars for which proper motion numbers exist, but what little motion there is suggests none of them are related physically to one another:

This is an erect image, so east and west are opposite of my sketch above as well as the photo above. Click to enlarge.

We’ll cover the last three multiple stars of CR 65 in the next post, along with a bonus that wasn’t on my list.  Hopefully we’ll have some good seeing — we’re going to need it for this group!

Clear Skies!  😎